Effects of Added Uranium on the Triboluminescent Properties of EuD4TEA

In 1888, Wiedemann and Schmidt defined triboluminescence (TL) as the emission of light produced by a mechanical action [1]. This property is present in about 50% of known crystals [1]. Of the hundreds of known triboluminescent materials, only a few are bright enough to be seen in daylight. One organic material that shows TL in daylight is europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA) [2–4]. This material shows 206% of the TL yield compared to ZnS:Mn when subjected to low energy impacts [3,5]. The first EuD4TEA material was synthesized by Hurt et al. [4]. Due to the high cost of anhydrous europium (III) chloride and the time required for purification, the authors in 2010 replaced the chloride salt with europium (III) nitrate hexahydrate [3]. This replacement enhanced the triboluminescent light yield by 82% and the material yield also increased due to the lack of washing required [3]. In 2011, the authors investigated the effect of various solvents on the TL of EuD4TEA [6]. Results from this study showed that solvents can make a difference in the amount of TL produced upon impact [6]. The large crystal sized samples made with acetone produced the most TL [6]. Surprisingly, however, the use of the expensive pure ethyl alcohol failed to produce the largest light yield [6]. Instead, the use of an inexpensive laboratory grade acetone as a solvent to make EuD4TEA crystals produced the most TL of all the solvents tested [6]. These results indicated that the synthesis of EuD4TEA is fully optimized. In order to increase the TL yield even more, impurities can be added to the EuD4TEA. This paper will explore the effects of adding uranyl acetate on the triboluminescent properties of EuD4TEA. This material was chosen for study due its strong luminescence. The uranyl acetate manufactured by Mallinckrodt, Incorporated has an activity of 0.2 μCi/g since depleted uranium was used to synthesize the acetate. The synthesis of uranium doped EuD4TEA was based on Reference [3]. The process began by pouring 75 mL of 95% denatured ethanol into a beaker. The ethanol was heated to 250°C and the stirring set to maximum (12). Then 4 mmol of europium (III) nitrate hexahydrate and the desired amount of uranyl acetate were added to the hot solution. The quantity of added uranium ranged from 0 to 100 mole percent that was determined by using the ratio of U/Eu. Once the uranium acetate and europium dissolved, 13 mmol of 1,3-Diphenyl-1,3-propanedione also known as dibenzoylmethane (DBM) was added to the hot solution. A funnel was then placed on top of the beaker and the solution was left for 20 minutes. After 20 minutes, the stirrer was removed and 14 mmol of triethylamine (TEA) was added. The solution was then kept aside to cool at ambient temperature. The EuD4TEA crystals that formed were filtered and air dried at room temperature. One interesting observation was that for low concentrations of uranium (<10%), the material was light yellow in color (common egg yolk). However, as the concentration of uranium increased, dark orange spots (“uranium pox”) started to form. A further increase in concentration caused the whole material to turn dark orange in color. It is likely that it becomes increasingly more difficult to incorporate the uranium inside the crystal structure. In addition, as the uranium concentration becomes larger, the fluorescence yield to decrease. There appears to be a direct relation between the reduction in fluorescence yield and sample color. A specially designed drop tower was used to measure the TL properties of each tetrakis material as described in Reference [5]. The experiment began by placing the sample material on the center of the Plexiglas plate. Next, a 130 g steel ball bearing is placed 42 inches (1.07 m) above the (from) sample and is then released. After each test, the drop tube is removed, the ball is cleaned, and the powder redistributed near the center of the target area [5]. To determine TL yield for each material, a photodiode is placed under the plexiglass plate. An amplifier increases the signal into an oscilloscope that records the resulting data in single sequence mode. Later, the signal is analyzed using a custom LabVIEW code that integrates the area under the curve [5]. Results show that the addition of four mole percent of uranium increases the initial triboluminescent yield from plain EuD4TEA by about 80%. However, gains in TL yield were found to decrease with time based on the emission of alpha particles by the depleted U in these samples. Fluorescence emitted by materials exposed to ionizing radiation from heavy charged particles follows the Birks and Black relation as shown in References [7] and [8]. These alpha particles break chemical bonds, thus reducing the radiative emission of fluorescence in EuD4TEA with added uranium. After 120 days, the TL emission was measured for the four mole percent sample. Results here showed the triboluminescent yield was reduced by about 20% over the initial value measured when the sample was synthesized. At this rate, it should take about 335 days for the TL yield to be reduced to half of its original value. It appears that adding uranium damages the EuD4TEA making it worthless as an additive to increase the fluorescence yield. However, this result opens up new potential for EuD4TEA to be used as a radiation sensor for heavy charged particles, like those that exist in the space environment. The so-called half brightness exposure time was 335 days, which is equal to a total integrated activity for alpha particles of 3 GBq. This total activity is relatively small which holds great promise for use as a low-level radiation sensor. Additional research is needed to further quantify these results. This poster will show the results of this research as well as the potential for EuD4TEA to be used as a radiation sensor.